1. Field of the Invention
The present invention relates generally to the protection of intellectual property, and more specifically, to a robust digital watermark that is encoded into a Direct Stream Digital (DSD) signal.
2. Description of the Related Art
When the compact disk (CD) was first developed, Pulse Coding Modulation (PCM) was the best available technology for encoding a digital audio signal to be recorded on a disk. The standard CD format has a 16-bit resolution and sampling frequency of 44.1 kHz.
As a result of advances made in professional recording techniques, the resolution and sampling rate of the standard CD format is no longer adequate to preserve the audio quality of the recordings being made at various digital studios.
FIG. 1A illustrates the typical audio spectrum of the standard CD signal. As shown in this figure, the signal band of standard CD strongly diminishes after 20 kHz.
Super Audio Compact Disk is a high-resolution audio CD format, which utilizes Direct Stream Digital (DSD®), instead of PCM, for recording audio signals. The DSD recording format directly records the 1-bit signal, output from a sigma-delta analog-to-digital (A/D) converter at a sampling rate of 2.8224 MHz. The sigma-delta A/D converters used in DSD include noise shaping filters to effectively shift the noise out of the audio band, i.e., shift the noise well above 20 kHz where it is inaudible.
DSD avoids the degradation in signal quality that may result from the decimation process and interpolation processes used in PCM. DSD recordings have an improved frequency response, which is broadened to 100 kHz. In addition, the DSD frequency response has a dynamic range of 120 dB when noise shaping filters of fifth (or higher) order are included in the sigma-delta A/D converter. FIG. 1B illustrates the audio spectrum of DSD signals.
Although human ears generally do not hear distinct tones over 20 kHz, the higher frequencies do contain audio information of a transient nature (i.e., the way sounds start and stop), which is perceived by the human brain and provides a more “natural” feeling to the music. By preserving the information in this high-frequency band (i.e., in the noise spectrum), DSD technology provides a more accurate impulse response and sharp transients, which is a significant reason for the superior audio quality of DSD recordings over the standard CD recordings.
Along with audio quality, another important issue that must be considered with respect to digital recording technologies is protection of the recorded content from illegal copying and distribution. One mechanism for protecting digitally recorded intellectual property (IP) is an encoded watermark.
A watermark comprises identification data embedded within digital content, which may specify the ownership and the intended recipient of the content, and help enforce restrictions on usage (e.g., copying and distribution) of the content. Watermarks can be used to identify illegal copies of digital content, detect illegal changes in the content.
Some watermarks are used as an attempt to prevent unlawful copying and distribution of digital IP. For instance, certain types of CD recorders may contain a special copy protection mechanism, which checks for a watermark within the content to be recorded. If a watermark is detected that indicates that the copying is not permitted or that the content has been illegally obtained, the mechanism disables the recording operation of the device.
Alternatively, the purpose of a watermark may be to identify the unlawful copying and distribution of content, in order to allow the rights owners to seek legal reparations from the infringing parties.
A watermark generally comprises a signal encoded into digital content that fulfills certain statistical properties, such that it can be extracted from the content in its original form. However, the addition of such a signal usually causes some degradation to the sound quality of digitally recorded audio signal. It would be advantageous to provide a watermark, whose presence does not cause any perceivable degradation in the sound quality of an audio recording.
It would further be advantageous to provide a watermark that is robust, i.e., difficult to remove or modify without severely affecting the quality of the audio signal in which it is embedded. Accordingly, the watermark must be placed in perceptually significant components (e.g., frequency bands) of the audio signal.